Place the potatoes in a medium saucepan and cover with cold water. Bring the water to a boil and cook until the potatoes are just tender. Drain. Meanwhile, in a medium bowl, combine the mustard and vinegar along with a pinch of salt and pepper. Whisk in the olive oil. As soon as the potatoes are cool enough to handle, cut them in half and toss them with the mustard mixture. Refrigerate. (This can be done in advance and stored in the refrigerator for up to 3 days.) When ready to serve, stir the celery, red onion, pickles and sour cream into the potato mixture. Adjust the salt and pepper to suit your taste.